Heart disease and its manifestations, including coronary artery disease, myocardial infarction, congestive heart failure and cardiac hypertrophy, clearly present a major health risk in the United States today. The cost to diagnose, treat and support patients suffering from these diseases is well into the billions of dollars. Two particularly severe manifestations of heart disease are myocardial infarction and cardiac hypertrophy.
Myocardial infarction, commonly known as a heart attack, is caused by a sudden and sustained lack of blood flow to the heart tissue, which is usually the result of a narrowing or occlusion of a coronary artery. Without adequate blood supply, the tissue becomes ischemic, leading to the death of cardiomyocytes (e.g. heart muscle cells) and vascular structures. The necrotic tissue resulting from the death of the cardiomyocytes is generally replaced by scar tissue, which is not contractile, fails to contribute to cardiac function, and often plays a detrimental role in heart function by expanding during cardiac contraction, or by increasing the size and effective radius of the ventricle, for example, becoming hypertrophic.
Cardiac hypertrophy is an adaptive response of the heart to virtually all forms of cardiac disease, including those arising from hypertension, mechanical load, myocardial infarction, cardiac arrhythmias, endocrine disorders, and genetic mutations in cardiac contractile protein genes. While the hypertrophic response is initially a compensatory mechanism that augments cardiac output, sustained hypertrophy can lead to dilated cardiomyopathy (DCM), heart failure, and sudden death. In the United States, approximately half a million individuals are diagnosed with heart failure each year, with a mortality rate approaching 50%.
Numerous signaling pathways, especially those involving aberrant calcium signaling, drive cardiac hypertrophy and pathological remodeling (Heineke & Molkentin, 2006). Hypertrophic growth in response to stress involves different signaling pathways and gene expression patterns than physiological hypertrophy, which occurs in response to exercise. Stress-mediated myocardial hypertrophy is a complex phenomenon associated with numerous adverse consequences with distinct molecular and histological characteristics causing the heart to fibrose, dilate and decompensate which, through cardiomyocyte degeneration and death, often culminates in heart failure. As such, there has been intense interest in deciphering the underlying molecular mechanisms and in discovering novel therapeutic targets for suppressing adverse cardiac growth and ultimately failure. Understanding these mechanisms is essential to the design of new therapies to treat cardiac hypertrophy and heart failure.
Metabolic syndrome is a combination of medical disorders that increase the risk of developing cardiovascular disease and diabetes. It affects one in five people, and prevalence increases with age. Some studies estimate the prevalence in the U.S. to be up to 25% of the population (Ford et al. (2002) JAMA, Vol. 287:356-359). People afflicted with metabolic syndrome are generally obese, sedentary, and have a certain degree of insulin resistance. Obesity is a leading preventable cause of death worldwide, with increasing prevalence in adults and children, and authorities view it as one of the most serious public health problems of the 21st century (Barness et al. (2007) Am. J. Med. Genet. A, Vol. 143A: 3016-34). Obesity can lead to reduced life expectancy and increased health problems, including heart disease, type 2 diabetes, sleep apnea, certain types of cancer, and osteoarthritis. Current therapies for metabolic syndrome and obesity focus on dieting and exercise with very few effective pharmaceutical interventions available. The effectiveness of diet and exercise in improving these conditions varies greatly among patients and tends to provide only a moderate degree of weight loss and improvement in symptoms. Therefore, there is a need for novel therapeutic approaches to treat metabolic disorders and prevent the subsequent development of cardiovascular disease and heart failure.
MicroRNAs have recently been implicated in a number of biological processes including regulation of developmental timing, apoptosis, fat metabolism, and hematopoietic cell differentiation among others. MicroRNAs (miRNAs) are small, non-protein coding RNAs of about 18 to about 25 nucleotides in length that are derived from individual miRNA genes, from introns of protein coding genes, or from poly-cistronic transcripts that often encode multiple, closely related miRNAs. See review by Carrington et al. (Science, Vol. 301(5631):336-338, 2003). MiRNAs act as repressors of target mRNAs by promoting their degradation, when their sequences are perfectly complementary, or by inhibiting translation, when their sequences contain mismatches.
MiRNAs are transcribed by RNA polymerase II (pol II) or RNA polymerase III (pol III; see Qi et al. (2006) Cellular & Molecular Immunology, Vol. 3:411-419) and arise from initial transcripts, termed primary miRNA transcripts (pri-miRNAs), that are generally several thousand bases long. Pri-miRNAs are processed in the nucleus by the RNase Drosha into about 70- to about 100-nucleotide hairpin-shaped precursors (pre-miRNAs). Following transport to the cytoplasm, the hairpin pre-miRNA is further processed by Dicer to produce a double-stranded miRNA. The mature miRNA strand is then incorporated into the RNA-induced silencing complex (RISC), where it associates with its target mRNAs by base-pair complementarity. In the relatively rare cases in which a miRNA base pairs perfectly with an mRNA target, it promotes mRNA degradation. More commonly, miRNAs form imperfect heteroduplexes with target mRNAs, affecting either mRNA stability or inhibiting mRNA translation.
Recently, signature expression patterns of miRNAs associated with pathological cardiac hypertrophy, heart failure and myocardial infarction in humans and mouse models of heart disease have been identified (van Rooij et al (2006) Proc. Natl. Acad. Sci., Vol. 103(48):18255-60; van Rooij et al., (2007) Science, Vol. 316: 575-579). Gain- and loss-of-function studies in mice have revealed profound and unexpected functions for these miRNAs in numerous facets of cardiac biology, including the control of myocyte growth, contractility, energy metabolism, fibrosis, and angiogenesis, providing glimpses of new regulatory mechanisms and potential therapeutic targets for heart disease. Remarkably, knockout mice lacking disease-inducing miRNAs are normal, but display aberrant responses to cardiac stress, suggesting the dedication of these miRNAs to disease-related processes rather than tissue homeostasis, and pointing to their potential as therapeutic targets. Thus, miRNAs represent potential novel therapeutic targets for the development of treatments for a variety of diseases, including cardiovascular diseases, obesity, diabetes, and other metabolic disorders.